3-methylbutene-1 polymers (also called 3-methyl-1-butene polymers) have been known as crystalline polyolefins having high melting points. For instance, Japanese Unexamined Patent Publications No. 182305/1982, No. 195704/1982 and No. 8708/1983 disclose processes wherein 3-methylbutene-1 is polymerized by means of titanium trichloride having a small aluminum content and an organoaluminum compound. While 3-methylbutene-1 polymers obtained by these conventional processes have high melting points, the mechanical strength of the molded products such as tear strength or impact strength and the stretchability are inadequate. Therefore, such products are not necessarily satisfactory as materials for various molded products or as materials for stretched or non-stretched films.
The present inventors have conducted extensive studies with an aim to overcome the above-mentioned various drawbacks, i.e. to improve the stretchability and the mechanical strength of the molded product, without impairing the heat resistance as the feature of 3-methylbutene-1 polymers. As a result, they have arrived at the present invention. Heretofore, such a 3-methylbutene-1 polymer composition and the process for its production have not been known.